A data write method in which an operating technique, what is called a Quick Pass Write method, is used has been proposed as a technique of improving reliability of the nonvolatile semiconductor storage device.
A verify operation in which, in addition to a verify voltage (hereinafter referred to as a “real verify voltage”) that is provided in a lower limit of a threshold voltage distribution set in each piece of data, another verify voltage (hereinafter referred to as a “preverify voltage”) that is slightly lower than the real verify voltage is used is performed in the QPW method.
A normal program operation is performed until a threshold voltage of a memory cell transitions to the preverify voltage, and a fine program operation in which a transition width of the threshold voltage is narrowed is performed after the threshold voltage exceeds the preverify voltage. As a result, the threshold voltage distribution of the memory cell can be prevented from being widened.
However, in the data write in which the QPW method is adopted, a processing time tends to be lengthened during the data write because many detection operations are required in the verify operation.